The magnetic storage layer of a magnetic recording medium usually has a structure in which minute magnetic particles are decoupled by nonmagnetic grain boundaries. In recent years, methods for structuring grain boundaries made of a nonmetallic material such as an oxide are being studied. Typical examples of such methods are introduced in IEEE Trans. Magn., vol. 36, p. 2393, 2000, and IEEE Trans. Magn., vol. 38, p. 1976, 2002. Such a grain boundary structure enhances independency of the magnetization direction of each magnetic particle and makes the unit of magnetization reversal in a magnetic storage layer smaller, so that it becomes possible to increase the recording density.
To further increase the recording density, besides making the unit of magnetization reversal in a magnetic storage layer smaller, it is also desirable to provide the film with thermal stability required to retain magnetically recorded information as well as to enable recording with a limited size of magnetic head field.
In the perpendicular magnetic recording method, the demagnetization fields from recorded bits do not act on the magnetic transition regions between recorded bits, and magnetized states resulting from recording are stabilized. The perpendicular magnetic recording method is therefore considered more advantageous in terms of high-density recording than the conventional longitudinal magnetic recording. Moreover, even when a thick magnetic film is used, a perpendicular magnetic recording medium, as compared with a longitudinal magnetic recording medium, can suppress deterioration of the recording and playback resolution, so that the perpendicular magnetic recording medium is considered more advantageous in terms of thermal stability. However, it has been reported that, even in a perpendicular magnetic recording medium, demagnetization fields substantially affect magnetization in areas away from the magnetic transition regions, particularly where the recording density is relatively low, causing the read output to largely decrease. Therefore, for perpendicular magnetic recording, it is also necessary to cope with thermal stability.
To improve the thermal stability of a perpendicular magnetic recording medium, it is effective to increase the magnetic anisotropy energy of magnetic particles, but doing so results in higher magnetic fields required for recording. Since the magnetic flux density that can be generated by a write head is limited, increasing the magnetic field required for recording makes it difficult to perform writing using the write head. As a result, the recording performance may deteriorate remarkably. The thermal stability can be increased also by enlarging magnetic particles of the perpendicular magnetic layer. Generally, however, when the magnetic particles are made larger, fine zigzag shapes of magnetic transition regions become larger and possibly increase medium noise.
As described above, enhancing thermal stability often deteriorates recording performance in high recording density regions.
FIG. 1 is a conceptual diagram showing a magnetization curve of the perpendicular magnetic layer of a typical perpendicular magnetic recording medium. In FIG. 1, three magnetic field parameters, i.e. magnetic saturation field Hs, coercivity Hc, and magnetic nucleation field Hn are also shown as parameters representing characteristics of the magnetization curve, along with an inclination α at coercivity Hc of the magnetization curve. To perform magnetic recording on a perpendicular magnetic layer requires a head magnetic field larger than Hs to be generated. To allow magnetized information recorded on the perpendicular magnetic layer to remain stable after the magnetic field is removed, Hn is required to be a positive value. Roughly speaking, therefore, when the value of (Hs-Hn) of a magnetic recording medium is smaller, that is, when the inclination α at around the coercivity Hc of the magnetization curve is larger, it is easier to write data on the magnetic recording medium and the magnetic recording medium is less easily demagnetized.
On the other hand, there is a tendency that the recording performance of a perpendicular magnetic layer is higher when its inclination angle α is smaller. Instances showing such a tendency are described, for example, in JP 2002-197636 A and JP 2005-251264 A. In each of the instances, optimizing the film formation process decreased the inclination angle α and, as a result, the signal-to-noise ratio of the playback signal of information recorded (on the film) correspondingly improved. These results are considered to be associated with a decrease in exchange interaction taking place between magnetic particles in the recording magnetic layer. Since magnetization direction became more independent between magnetic particles and small magnetic domains became easier to form that the recording performance improved. The decrease in the inclination angle α means that, as a result of weakening of the exchange interaction, it became less likely for the magnetization directions to be uniform between magnetic particles. This invites an increase in the recording fields required for sufficient recording and a decrease in thermal stability of the saturation state of magnetization.
It is therefore difficult to obtain, just by decreasing the exchange interaction between magnetic particles, a perpendicular magnetic recording medium which shows superior recording performance while offering both ease of recording and thermal stability. In order to realize desired recording performance while suppressing the decrease in the inclination angle α, one must tolerate a certain degree of exchange interaction and uniformizing the magnetization switching fields of the perpendicular magnetic layer. When the exchange interaction is excessive, however, the write/read characteristics inevitably deteriorate. Uniformizing the magnetization switching fields is also difficult to achieve.